Alkali-metal electrochemical systems generally require nonaqueous electrolytes. Such electrolytes usually comprise organic solvents and electrolytically active alkali-metal salt solutes, as taught for example by U.S. Pat. No. 4,104,450, issued Aug. 1, 1978.
The particular electrolyte used in these alkali-metal electrochemical devices is an extremely important factor in obtaining efficacious performance.
The choice of a proper and effective electrolyte is not always predictable in advance. Sometimes, as in the case of the present invention, the choice of electrolyte is completely contrary to established teachings.
It has been observed that for alkali-metal batteries the discharge rate will be improved with the amount of solute which can be dissolved in the electrolyte solution.
It has been discovered that alkali-metal complex anion salt solutes of the general formula suggested by the aforementioned patent and hereinafter described have been most useful in this regard. These complex anion salt solutes are not only more soluble in typical organic solvents such as dioxolane, but also provide improved anodic stability, i.e., the electrolyte is oxidatively stabilized.
Electrolytes having anodic stability are most desirable in alkali-metal battery systems, because they allow for improved storage and shelf life.
Spontaneous or premature discharge of the battery is prevented.
In general, it has been demonstrated in the U.S. Pat. No. 4,104,540 that heteroatom-containing substituents which are bonded to the core element of the complex anion salt through the heteroatom itself provide adequate anodic stability. Such salts, however, many not simultaneously provide sufficient solubility of the salt in the solvent.
It has now been discovered, with reference to this invention, that the solubility of a complex anion of salt may be further improved, and the anodic stability of the electrolyte maintained or further enhanced, by bonding the heteroatom groupings to the core element via a ring carbon atom. The choice of this electrolyte solute is not obvious.
Many published reports demonstrate that when such a heteroatom grouping is bonded to boron through a ring carbon atom, the resulting salt becomes extremely reactive. This reactivity leads to the subsequent destruction of the complex anion system. Naturally such reactivity is not desirable when formulating electrolytes, particularly where improved anodic stability is sought.
Two published reports which demonstrate the aforementioned reactivity of carbon-bonded heteroatom groupings to boron can be found in the articles to: A. B. Levy, J. Org. Chem., 43, 4684-5 (1978); and A. Suzuki, N. Miyaura and M. Itoh, Tetrahydron, 27, 2775-83 (1971).
In addition, another study has shown that alkali-metal salts of complex anions substituted with carbon-bonded thienyl (heteroatom) groupings generally exhibit low solubilities. This teaching can be found in the Ph.D thesis of G. E. Paley, entitled "Studies in Tetrathienylborates--Their Synthesis and Analytical Properties", Loyola University (1979), University Microfilms.
Obviously, the low solubilities predicted for these salts suggest that they would be a poor choice as solutes in electrolytes of alkali metal batteries.
Therefore, it is not obvious that salts of this type, which are the subject of this invention, are useful in formulating electrolytes.